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Creating ARM-based Cloud RAN wireless base station

Posted: 27 Feb 2014 ?? ?Print Version ?Bookmark and Share

Keywords:Cellular service? Cloud radio access network? Cloud RAN? base band? MPU?

Cellular service providers are in the quest for cost-effective, scalable ways to manage their networks profitably. Cloud radio access network (Cloud RAN) technology is gaining traction with service providers as an efficient means of processing wireless network signals by virtualizing base band processing onto large server farms and ultimately reducing costs.

This article describes a novel architecture for base band processing using ARM's Cortex A57 processors for use in mobile wireless base stations in conjunction with our modem processing unit (MPU), a real-time, reconfigurable platform that allows for the implementation of a wide variety of communication standards in Cloud RAN as a tightly integrated co-processor to general-purpose computers. This approach reduces power consumption, increases overall network throughput, and decreases CAPEX and OPEX by offloading tasks from older base stations that are expensive to operate.

Figure 1: Cloud RAN architecture, fully centralized solution [1].

Radio access networks
Conventional base stations are the core of wireless network RANs. They include unified RF units and base band processing units positioned at the base station site. From an operator's point of view, this approach has significant limitations [1]. Each base station connects to a fixed number of sector antennas outfitted for peak voice and data demand at their coverage region. With this approach, it is nearly impossible to improve system capacity since interference mitigation techniques are difficult to employ. Furthermore, the base stations are built on proprietary platforms, and as such, they are expensive to construct, maintain and operate.

Cloud RAN is a compelling alternative approach. It is composed of three main parts: distributed radio units, antennas, and remote radio unit (RRUs) located at the remote site, a base band unit (BBU) pool comprised of high-performance general purpose processors (GPP) located in a data centre, and a high-bandwidth, low-latency optical transport network that connects the RRUs and the BBU pool. The Cloud RAN approach not only reduces the construction costs (assuming fibre optical backhaul already exists) and operation costs of each base station facility, it also allows for dynamic reallocation of virtualized processing resources from one base station to another when utilisation shifts throughout the day and week.

A main technical challenge of Cloud RAN is the BBU pool implementation. According to a recent study [1], a centralized BBU pool of a medium-sized dense urban network (25 km2) should support about 100 base stations (300 sectors) while each BBU should meet the high throughput and low latency requirements of a modern wireless communication standard with the goal of executing the all software layers on GPPs. In addition, the BBU pool should be highly power-efficient in order to show a real decrease of power consumption compared to conventional systems that use efficient base stations.

Successful attempts to develop full base stations on a pure GPP platform are reported in recent studies [1, 2, 4]. However, these studies show that in spite of using state-of-the-art platforms and innovative techniques, such platforms are not as optimised as dedicated SoC platforms when it comes to executing intensive physical layer tasks, such as Turbo decoding, FFT, and large-scale MIMO decoding.

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